Heat exchangers, such as the steam generators of pressurized water nuclear reactors, comprise a tube plate of great thickness, upon which the tube bank is mounted.
The tubes of the bank pass through the plate from its inlet face in contact with the primary fluid to its outlet face in contact with the secondary fluid.
In the steam generators of pressurized water nuclear reactors, the primary water enters the water chest, one of the walls of which is formed by the inlet face of the tube plate, with which the ends of the tubes are flush.
Feed water is introduced into the part of the steam generator located above the tube plate and comes into contact with the tubes of the bank, within which the primary fluid circulates. The heat of the primary fluid permits the vaporization of the feed water.
The outlet face of the tube plate is covered by a stainless casing.
After the assembly of the tube bank, it is necessary to inspect the state of the tube plate, and particularly to inspect the zone of this tube plate located beneath the casing, since defects can appear in this zone.
The inspection must be performed from the water chest of the steam generator, i.e., from the inlet face of the tube plate.
Inspection probes must therefore be introduced successively into each of the tubes passing through the tube plate in order to perform the inspection of the zone located close to the outlet face of the tube plate, around the relevant tube.
The probes must therefore be capable of moving in translation within the tue over the total span of the tube plate, this span being of the order of 600 mm, and in rotation about the axis of the tube, in order to perform the inspection of the portion of the tube plate located around the tube.
Inspection devices for the tubes of a steam generator are known, consisting of an assembly capable of moving in translation in the tube by virtue of a cable possessing sufficient rigidity to exert a thrust upon the inspection device. This device can be introduced into each of the tubes successively, and comprises a support equipped with fixing means to the inlet face of the tube plate.
Since the objective is to facilitate the passage of the inspection device along the entire length of the tube, which is bent in a U shape at its top end, the active part of the device comprising the inspection probe exhibits guide means permitting this active part to follow the curvature of the tube.
However, it is impossible in this way to ensure very good centering of the probe in the tube.
In the case of the inspection of the defects under the casing of the tube plate, such devices are not satisfactory, because the prime consideration is a perfect centering of the probe at the level of the outlet face of the tube plate, since the probe is then set in rotation about the axis of the tube in order to inspect the zone of the tube plate located at the periphery of the tube.
Moreover, in the prior art devices, the means of moving the probe must be placed in the prolongation of the axis of the tube so that the thrust permitting the movement of the inspection device can be exerted in the correct direction.
For the inspection of the tube plate, at the level of the peripheral tubes which are very close to each other, it is impossible to ensure an appropriate arrangement of the means of moving the probe. This is due to the domed shape of the end of the steam generator forming the outer wall of the water chest.